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The café wall illusion: the horizontal lines are parallel, despite appearing to be at different angles to each other

An illusion is a distortion of perception. The brain arranges, sorts, and organises data from the senses. Normally the system works well. Sometimes it does not, and we see illusions. In general, they are shared by most people in the same situation.

Illusions can happen with all five senses (taste, touch, sight, smell, and hearing), and some involve the way information from two senses is put together. Some illusions happen because of disorders, but generally, all normal people can sense the same illusion. An illusion is different from a hallucination; a halucination is sensing something which is not real, but an illusion is interpreting what we sense wrongly.

Optical illusions

Optical illusions are the best known and understood. Because they are illusions common to most people, it follows that they are a product of the visual system. The visual system, from the retina to the visual cortex, is what is common to most humans.

Auditory illusions

These are illusions which are heard. The person listening hears sounds that aren’t really there, or sounds that can’t really happen. A Shepard tone is an auditory illusion.

Touch illusion

They are when a person feels something that isn’t there, or something that shouldn’t be there but feels like it is. This happens sometimes when a person loses his arm. The person still feels like the arm is there. They might want to scratch the arm, or the arm might hurt them. This can happen with any limb of the body, and it is called the phantom limb.